The art of porous chromium plating has been in widespread use for a number of years both in decorative plating and industrial or so-called "hard chromium" plating. As evolved over the years, three principal control factors are employed in hard chromium plating: (1) the manner of deposition of the chromium plate onto the metal surface; (2) the etching treatment; and (3) the finishing of the resultant plated surface. Conventionally, the plating operation requires close control of the composition of the bath, its temperature and current density. The etching step may be done by any of several methods including chemical or electrochemical treatments and the etched effect may also be produced by mechanical means. Several types of finishing are customarily employed such as polishing, lapping or honing, and it is generally considered that the amount of plating removed and its rate of removal affects the porosity of the plate.
Typically, porous chromium plate in which the plating is formed with fissures, pits or microcracks has been used in articles in which wear and operating characteristics are at a premium, such as, in the production of piston rings and cylinder walls, since the porous chromium plate is known to act as an extremely good bearing surface having improved load-carrying and lubricating characteristics. As the chromium is deposited in the plating operation, cracks of microscopic size are produced almost in a random pattern resulting from the stresses created in the deposit. As the stress develops until it reaches the ultimate strength of the chromium deposit, the resultant fracture relieves the stress thereby leaving fine cracks in the plating. The size and type of the porosity, or microcracking, can be controlled by various means and for example an etching step following the initial plating step is known to widen or expand the cracks formed. Generally, as the etching proceeds, the fissures or cracks initially deepen and widen; however, eventually an equilibrium is established between the rate of dissolving in the cracks or fissures and from the surfaces so that there is relatively little, if any, further increase in the depth of the fissures or pores. Beyond this point, the plate continues to dissolve without any apparent increase in the depth of channels unless some preliminary treatment is employed to protect the surface against erosion or eating away during the etching process. Thus one object of the present invention is to provide a means of controlled microcracking and specifically of selective expansion of the microcracks by etching without in any way altering or affecting the external surface of the plate.
Moreover, in microcracking a hard chromium plate, factors favoring a desirable crack pattern include relatively high bath temperatures, low chromic acid concentration, relatively high fluoride content and thickness. A conventional approach in chromium plating has been the formation of the plate in multiple layers by application of two successive baths, the first to obtain coverage and thickness and the second to create the desired pattern of cracks. However, it has been found in accordance with the present invention that the initial crack pattern may be formed in a single chromium plating operation followed by heat treatment as a preliminary to reverse etching to expand the crack pattern, and in this relation, it is desirable to carry out the plating operation for a sufficient length of time to insure a relatively thick plating for example in the range of 0.002 to 0.004 inches in thickness.
In recent years it has been found that the lubricant or low-friction characteristics of hard chromium plated surfaces can be greatly enhanced by the injection or insertion of perfluorocarbon resins such as polytetrafluoroethylene into the fissures or pores formed in the plated surface. For example, U.S. Pat. No. 3,279,936 to Forestek is representative of such a process specifically wherein a porous surface such as a microcracked chromium plated surface has applied thereto non-fused particles of polytetrafluoroethylene which are mechanically locked into the fissures or cracks formed in the surface. Generally, in accordance with the Forestek patent, the approach taken is to heat the plated surface above a predetermined temperature level to enlarge the fissures by thermal explosion and to mechanically force unfused particles into the pores. It is theorized that when the surface is then cooled, the size of the pores or fissures is reduced to mechanically lock the particles in place by an interference fit. However, such process presupposes that the powdered resin is locked physically in place within the pores or fissures, and therefore specific limits are placed on the temperatures to which the surface may be heated in order to avoid fusion of the polytetrafluoroethylene particles, specifying that the maximum temperature limit be 650.degree. F and preferably less than 500.degree. F in the range of 250.degree. to 400.degree. F. In practice, however, it has been found that the degree of expansion induced merely by heating to a limited extent is indeed very slight making it difficult to assure uniform and complete filling of the fissures or pores with unfused particles. Moreover, the degree of contraction of the fissures once cooled is not such as to uniformly lock the particles in place particularly when the surface is subjected to heat expansion or repeated wear over an extended period of time.